The present invention pertains to the field of processing unsteady pressure signals such as in the field of sonar, and more particularly to the field of measurements of the speed of a fluid when the direction of fluid flow is known, such as when the fluid flow is confined to a pipe.
A nonintrusive measurement of flow velocity is desirable in many situations, including for example, in drilling for oil and natural gas, where the flow velocity of the production fluid containing the oil and gas being pumped from a formation must be monitored and controlled for efficient extraction. Fluid flow velocities in such applications are typically on the order of 10""s of ft/sec through conduits (pipes) of less than 6xe2x80x3 in inner diameter.
The term xe2x80x9cflow velocityxe2x80x9d is here used to mean average mixture velocity in the conduit. The flow velocity is related to the volumetric flow rate through the cross sectional area of the duct (i.e. volume flow rate=flow velocity times cross sectional area).
Standard flow measurement devices according to the prior art include orifice plates and venturis. Such devices are intrusive and require the measurement of pressure difference at two locations in the flow. The pressure difference is then related to flow rate. Other types of meters include turbine meters that spin in the flow. Such a meter relates the speed of rotation to the flow rate. Ultrasonic meters typically measure the difference in transit time with and against the flow within a pipe. Such a meter relates the difference in the transit times to the flow rate. All of these meters are either intrusive or require electronics.
Although the prior art teaches many methods to measure flow, both intrusive flow meters as well as flow meters based on non-intrusive methodologies, such as by measuring the time for an ultrasonic signal to propagate round trip between two sensors, for many applications what is needed is a method for determining flow velocity that is non-intrusive and at the same time avoids use of techniques such as measuring the time for an ultrasonic signal to propagate round trip between two sensors.
Accordingly, the present invention provides a method and corresponding apparatus for measuring the flow velocity of a fluid in an elongated body, the fluid traversing the elongated body substantially along the longest axis of the elongated body, the fluid bearing a vortical disturbance at a velocity correlated to the flow velocity, the vortical disturbance causing fluctuations in the pressure of the fluid near to the disturbance, the method comprising the steps of: providing an array of at least two sensors disposed at predetermined locations along the elongated body, each sensor for sampling the pressure of the fluid at the position of the sensor at a predetermined sampling rate; accumulating the sampled data from each sensor at each of a number of instants of time spanning a predetermined sampling duration; and constructing from the accumulated sampled data at least a portion of a so called k-xcfx89 plot, indicative of a dispersion relation for the propagation of acoustic pressures emanating from the vortical disturbances, the constructing of the k-xcfx89 plot based on correlating the sampled data provided by at least one of the sensors with the sampled data acquired by at least one other of the sensors, and by autocorrelating the sampled data acquired by each sensor.
In a further aspect of the invention, the method further comprises the steps of: using a feature extraction technique to identify a convective ridge in the k-xcfx89 plot; determining the orientation of the convective ridge in the k-xcfx89 plot; and determining the flow velocity based on a predetermined correlation of the flow velocity with the slope of the convective ridge of the k-xcfx89 plot.